1. Field of the Invention
The present invention relates to a mobile communication system that comprises a plurality of wireless base stations, a network that communicably connects the wireless base stations and a wireless network terminal that communicates wirelessly with the wireless base stations. More particularly, the present invention relates to improvements in a wireless base station that is contained in such a mobile communication system. The present invention can be applied to a Cellular Mobile Telephone System, PHS (Personal Handyphone System), or the like, for example. In addition, the present invention can be applied to wireless LAN (Local Area Network) that employs PDA (Personal Digital Assistance) and mobile computers, or the like, as communication terminals, to wireless communication systems based on Bluetooth® and digital wireless communication systems based on Zigbee®, or the like.
2. Description of Related Art
Mobile communication systems comprise a plurality of wireless base stations that communicate with wireless network terminals and wired networks that communicably connect the wireless base stations. Wireless base station being communicably connected to the wireless network terminal is called an ‘access point’.
The radio wave strength when the wireless network terminal and wireless base station are communicating is desirably sufficiently large. When the radio wave strength is smaller than a predetermined value, the wireless network terminal is unable to communicate with the wireless base station. Therefore, the wireless network terminal searches for an access point at which the radio wave strength reaches a maximum when the wireless network terminal is communicably connected to the wired network. Further, when the radio wave strength is smaller than a predetermined threshold value in accordance with the movement of the wireless network terminal, the wireless network terminal switches the access point with which the wireless network terminal is communicably connected. The switching is known as handover. Handover technology is disclosed in Japanese laid-open application publication No. 2001-258058, for example.
When performing handover, the wireless network terminal wirelessly transmits a probe request signal. The wireless base stations that receive a probe request signal transmit probe response signals to the wireless network terminal. In many cases, the wireless network terminal receives the probe response signals from a plurality of wireless base stations. The wireless network terminal measures the radio wave strength of the received probe response signals and selects the signal with the largest radio wave strength. Further, the wireless network terminal selects the wireless base station that transmitted the probe response signal with the largest strength as the new access point.
As mentioned above, the wireless network terminal measures the strength of all the received probe response signals. Therefore, the more wireless base stations that transmit a probe response signal, the longer then time required for the selection processing. When the processing time is very long, there is the risk that the radio wave strength between the access point and the wireless network terminal will fall below than the radio wave strength required for the communication connection before handover is complete. When the radio wave strength is inadequate, the communication connection is broken.
The wireless network terminal always selects the wireless base station corresponding with the largest signal even when a plurality of probe signals with slightly different radio wave strengths are received. As a result, when a plurality of wireless base stations is placed in the vicinity, there is the possibility of a concentration of a multiplicity of wireless connections with a small number of wireless base stations. The phenomenon whereby only the processing load of some of the wireless base stations increases brings about a deterioration of the operating efficiency of the network.
Some kinds of the mobile communication system such as PHS uses information indicating the position of each access point when handover is performed. A technology for registering the positions of the access points is disclosed in Japanese laid-open application publication No. 2003-174665, for example. This technology identifies the positions of access points by using a GPS (Global Positioning System). By using a GPS, registration can be automatically performed in a short time. On the other hand, the use of a GPS involves a huge cost increase.